Re: signals

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From: Shane Sendall (
Date: Thu 26 Sep 2002 - 15:22:38 BST

Hi Esha,
I haven't as such worked on activity diagrams, but I can offer my 
understanding of the relationship between signals and events (and 
stimuli) in the context of statemachines.
My understanding (of UML 1.4 spec) is that everything sent between 
instances (of actors/objects, etc.) are stimuli. Stimuli are caused by 
send or call actions.
Considering just the case of Signals, a send action is executed on the 
source instance, and consequently, a stimulus is created according to 
the related Signal (its just the specification of the stimulus that 
defines the signature; like a class for an object) and delivered to the 
target instance. The Reception at the target side determines whether the 
stimulus can be accepted (i.e., if it is supported by the classifier); 
if it is accepted by the Reception, then a SignalEvent instance is 
generated and placed on the state machine's event queue ready for 

I hope that helps.

Best of luck,

- Shane Sendall - Swiss Federal Institute of Technology in Lausanne -
- email : -
- phone : ++41 21 693 66 89         fax : ++41 21 693 50 79         -

Esha Ray wrote:

> Hi!
> I am student at UMass, Amherst and I am working with LASER(Lab for 
> Advanced Software Engineering Research). I am working with activity 
> diagrams and tring desperately to understand its semantics. We are 
> really stuck with signals right now. The UML user guide, reference 
> manual give very ambigous statemnets about signals. They also say that 
> acticity diagrams do not have events but only signals. I am not clear 
> on teh exact semantics about signals. Signals are also classifiers and 
> we can have a class diagram on the hierarchy of signals int he system. 
> Can anyone please tell me what exactly signals do in an activity 
> diagram and how are they interpreted.
> -Esha

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